1. Field of the invention
The present invention relates to superconducting material, more particularly, it relates to a new superconducting material which possesses a higher critical temperature of superconductivity and a reduced discrepancy between the critical temperature and the onset. temperature where the phenomenon of superconductivity is initially observed and which show enough stability in superconducting properties, as well as a process for preparing the same in a form of a block or in a form of a thin film.
2. Description of the related art
Under the superconducting condition, the perfect diamagnetism is observed and no difference in potential is observed for an electric current of a constant finite value, and hence, a variety of applications of superconductivity have been proposed in a field of electric power transmission as a means for delivering electric power without loss.
The superconductivity can be utilized in the field of electric power applications such as MHD power generation, power transmission, electric power reservation or the like; in the field of transportation; for example magnetic levitation trains, or magnetically propelling ships; in the medical field such as high-energy beam radiation unit; in the field of science such as NMR or high-energy physics; high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like, or in the field of fusion power generation.
In addition to the abovementioned electric power applications, the superconducting materials can be used in the field of electronics, for example, as a Josephson device which is an indispensable switching device for realizing a high-speed computer which consumes very reduced power.
However, their actual usage has been restricted because the phenomenon of superconductivity can be observed only at very low cryogenic temperatures. Among known superconducting materials, a group of materials having so-called A-15 structure show rather higher Tc (critical temperature of superconductivity) than others, but even the top record of Tc in the case of Nb.sub.3 Ge which showed the highest Tc could not exceed 23.2 K. at most. This means that liquidized helium (boiling point of 4.2 K.) is only one cryogen which can realize such very low temperature of Tc. However, helium is not only a limited costly resource but also requires a large-scaled system for liquefaction. Therefore, there has been a strong demand for other superconducting materials having higher Tc. However no material which exceeded the abovementioned Tc had been found from studies over the past ten years.
The possibility of an existence of new types of superconducting materials having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189].
This new oxide type superconducting material is [La, Ba].sub.2 CuO.sub.4 or [La, Sr].sub.2 CuO.sub.4 which are called the K.sub.2 NiF.sub.4 -type oxides having a crystal structure which is similar to known perovskite type oxide. The K.sub.2 NiF.sub.4 -type oxides show such higher Tc as 30 to 50 K., which are extremely higher than the known superconducting materials, and hence it becomes possible to use liquidized hydrogen (b.p.=20.4 K.) or liquidized neon (b.p.=27.3 K.) as a cryogen which can bring them to exhibit the superconductivity.
It was also reported in the parer that C. W. Chu et al discovered, in the United States of America, another type of superconducting material having the critical temperature of in the order of 90 K. in February 1987, and hence the possibility of existence of high-temperature superconductors have burst the scene.
Generally, the case of the abovementioned superconducting materials, there is a large discrepancy or difference between the onset temperature "Tc" (from where superconducting phenomenon started to be observed) and the critical temperature "Tcf" (from where resistivity vanishes completely and hence the complete superconductivity is realized). Therefore, in a practical use, these superconductors must be cooled down to a temperature which is fairly lower than the onset temperature "Tc", it is requested to reduce the abovementioned discrepancy or difference between "Tc" and "Tcf".
Furthermore, superconductivity breaks down in a strong magnetic field. The critical field "Hc" which is a definite value of magnetic field from where superconductivity breaks is dependent on temperature. Specifically both the critical temperature "Tc" and the critical magnetic field "Hc" are critical factors when superconductors are applied for practical uses. Therefore, it is also requested to improve the stability of superconductivity at the critical magnetic field.
The inventors of the present invention tried to solve the abovementioned problems by adding additional elements to the room-temperature superconductors which were discovered recently. It was expected that the addition of specified elements would improve a variety of properties of the superconducting materials so as to obtain higher critical current density and the stability of the critical current density under a strong magnetic field.
Experiments carried out by the present inventors revealed that the addition of one or more than one elements selected from a group consisting of V, Ta, Nb, Cr, Ga, In, Cd, Sn, Tl, Pb, Mo, W and Zn, to the compound oxide type superconducting materials, bring improvement in the properties of superconductivity, so as to obtain superconducting materials which exhibit a reduced discrepancy or difference between "Tc" and "Tcf" and whose superconductivity is not broken under stronger magnetic field.
Therefore, an object of the present invention is to solve the abovementioned problems and to provide a new superconducting material which exhibits a reduced discrepancy or difference between "Tc" and "Tcf" and whose superconductivity is not broken under stronger magnetic field, as well as to provide a process for producing the same in the form of bulk or block and in the form of a thin film.